"debatable, subject to discussion," by 1650s, from moot case (1570s), earlier simply moot (n.) in the specialized sense "discussion of a hypothetical law case" (1530s) in law student jargon. The reference is to students gathering to test their skills in mock cases.
1530s, "to mock" (transitive, now obsolete), from French alluder or directly from Latin alludere "to play, make fun of, joke, jest," also of waves lapping the shore, from assimilated form of ad "to" (see ad-) + ludere "to play" (see ludicrous). Meaning "make an indirect reference, point in passing" is from 1530s. Related: Alluded; alluding.